[Biological basis of orthodontic tooth movement].

The effect of orthodontic therapy is dependent of the biological possibilities and limitations of the dento-alveolar complex. Biomechanical effects determine the first phase of tooth movement. In the second phase hyalinisation occurs in almost all cases. Elimination of the hyalinised tissue is associated with undermining bone resorption. Next, 'real' tooth movement starts. At the pressure side the normal structure of the periodontal ligament is destroyed and so is the tooth attachment. At the tension side deposition of trabecular bone is found and the tooth attachment remains. The regulation of these processes is still not completely understood, but cytokines and growth factors play an important role. The biological system does not react according to a simple dose-response relation and large individual differences in susceptibility of the system exist.